This invention concerns desiccant-coated substrates and particularly desiccant-coated substrates that are useful in air treatment systems, for example, in heating, ventilation, and air conditioning ("HVAC") systems, and methods for making them.
Desiccants, their properties, and their uses (for example, in air treatment) and standards for air treatment and air quality are well-known. See, e.g., U.S. Pat. Nos. Re.29,932; 2,723,837; 2,926,502; 3,009,540; 3,009,684; 3,024,867; 3,024,868; 3,125,157; 3,266,973; 3,338,034; 3,528,224; 3,666,007; 3,844,737; 3,889,742; 4,012,206; 4,014,380; 4,021,590; 4,025,668; 4,036,360; 4,040,804; 4,081,024; 4,093,435; 4,109,431; 4,113,004; 4,130,111; 4,134,743; 4,140,458; 4,162,934; 4,172,164; 4,180,126; 4,222,244; 4,246,962; 4,255,171; 4,290,789; 4,325,220; 4,341,539; 4,346,051; 4,365,979; 4,382,807; 4,402,717; 4,431,456; 4,432,409; 4,449,992; 4,460,388; 4,484,938; 4,505,976; 4,527,398; 4,529,420; 4,540,420; 4,582,129; 4,594,860; 4,595,403; 4,635,446; 4,680,248; 4,723,417; 4,729,774; 4,747,346; 4,769,053; 4,808,505; 4,810,609; 4,871,607; 4,875,520; 4,886,769; 4,911,775; 4,919,899; 4,924,934; 4,948,392; 4,959,970; 5,002,116; 5,026,531; 5,045,295; 5,052,188; 5,120,694; 5,122,403; 5,125,195; 5,140,450; and 5,148,374; East Germany Patent No. 133855; Japan Application Nos. 53-36359 and 52-83907; EPO Application No. 0 030 863; and non-patent literature, including "EXCLU-SIEVE.TM.--Total Energy Recovery Systems--The Semco Air System," SEMCO Mfg., Inc., 8 pages (1991); "EXCLU-SIEVE.TM.--Heat Wheel Retrofit Program--The Semco Air System," SEMCO Mfg., Inc., 6 pages (1991); "EXCLU-SIEVE.TM.--Design and Selection Manual--The Semco Air System," SEMCO Mfg., Inc., 11 pages (1991); "EXCLU-SIEVE.TM.--Packaged Energy Recovery Systems--The Semco Air System," SEMCO Mfg., Inc., 33 pages (1991); "SEMCO EXCLU-SIEVE.TM.--Technical Bulletin 509," SEMCO Mfg., Inc., 2 pages (1991); "SEMCO EXCLU-SIEVE.TM.--Application Bulletin 508," SEMCO Mfg., Inc., 2 pages (1991); "SEMCO EXCLU-SIEVE.TM.--Application Case History Bulletin 507," SEMCO Mfg., Inc., 2 pages (1991); "SEMCO EXCLU-SIEVE.TM.--Application Case History Bulletin 506," SEMCO Mfg., Inc., 2 pages (1991); "Air exchanger eliminates cross contamination in animal lab," reprint from Consulting-Specifying engineer, 1 page (January 1990); "IAQ and Office Buildings: An EXCLU-SIEVE.TM. Solution," ASHRAE Journal's Sipplier Capabilities Supplement," pages S-44 and S-45 (August 1990); "Affordable Fresh Air is Now a Reality with EXCLU-SIEVE.TM. Total Energy Recovery," 1-page advertisement, SEMCO Mfg., Inc.; "A SEMCO EXCLU-SIEVE.TM. Retrofit . . . the workable solution," 1-page advertisement, SEMCO Mfg., Inc.; "EXCLU-SIEVE.TM. Design Solutions: Animal and Chemical Research Laboratories," Bulletin 504, Issue 1, SEMCO Mfg., Inc., 4 pages (July 1989); "Indoor Air Quality--A Fresh Solution," 1-page advertisement, SEMCO Mfg., Inc.; The Dehumidification Handbook, published by Cargocaire Engineering Corporation, 103 pages (copyright 1982, fourth printing November 1984); C. Bayer et al., "Results of Chemical Cross-contamination Testing of a Total Energy Recovery Wheel--Phase I," Georgia Institute of Technology, 8 pages (Jun. 5, 1991); "Union Carbide Molecular Sieves," page 4; "Molecular Sieves Manufactured by Davison Chemical," page 6; "Ethanol Drying Using Davison Molecular Sieves," Davison Chemical Division of Grace, page 3; "Molecular Sieves--SILIPORITE," page 4; "Davison Silica Gels," Introduction to Silica Gel and Silica Gel Application Guide (3 pages); "Davison 5A Molecular Sieves," Davison Chemical Division of Grace, 4 pages; Energy Recovery Equipment and Systems, SMACNA, Inc., page 6.5; D. W. Breck, Zeolite Molecular Sieves, pages 3,4, and 636; Methods of Dehumidification, Cargocaire Engineering Corporation handbook, pages 3-17 and 3-18; ASHRAE.TM. STANDARD 62-1989--Ventilation for Acceptable Indoor Air Quality, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., pages 1, 6-12, 15, 23, 24 (1989); C. Downing, "Tech Brief#15--Desiccant Air-Conditioning," Industrial Energy Extension Service of Georgia Tech, 4 pages (1989); "Finally. Superior Technology Makes Quality Indoor Air Affordable," SEMCO Incorporated, 1-page advertisement; Hawley's Codensed Chemical Dictionary, entry for "molecular sieve," pages 792-793 (11th edition 1987); "Senex Enthalpy Recovery Technical Manual," Cargocaire Engineering Corporation, 27 pages; and "Senex Energy Recovery--Cargocaire Bulletin 3315," Cargocaire Engineering Corporation, 6 pages. All of the documents identified and/or discussed herein, including all of the foregoing documents, are incorporated herein in their entirety for all purposes.
Methods are known for adhering particles of desiccant (e.g., molecular sieve particles, silica gel particles) to substrates to form desiccant-coated substrates used for air treatment, for example, heat and/or moisture recovery wheels that can be used in HVAC systems. Such wheels include total energy recovery (or enthalpy) wheels, which remove heat and moisture from one airstream and transfer them to another airstream, and dehumidification wheels, which transfer a significant amount of moisture while attempting to minimize heat transfer from one airstream to another. For example, it is known to make an enthalpy wheel, which has a thin (one-thousandth of an inch, i.e., 1 mil) layer of desiccant coating on each of the two major faces of its foil-like substrate, by saturating molecular sieve particles with water, dispersing them in an organic solvent containing a polyurethane binder composition to form a slurry, coating the slurry onto one major face of an aluminum foil substrate using a Rotogravure printing-type process, heating the composite sufficiently to set the binder to adhere the particles to the substrate and to cause the water to vaporize to prevent the binder from occluding the pore openings of the desiccant particles, repeating those steps to adhere a layer of desiccant particles to the other major face of the substrate, and then forming the wheel from the final composite. See also U.S. Pat. Nos. 3,338,034; 4,036,360; 4,769,053; 5,052,188; and 5,120,694.
U.S. Pat. No. 3,338,034 concerns adsorbent-coated thermal panels, specifically non-porous panels coated with thin layers of gas adsorbent adapted for rapid heating and cooling. The panels may be made of metal, preferably aluminum, stainless steel, or copper, and zeolite molecular sieves are preferred (column 2, lines 12-41). Preferably the adsorbent is bonded to the panel wall using an inorganic binder (e.g., clays) substantially free of any organic binder (column 3, lines 9-49). After the adsorbent-binder mixture has been applied to the surface of the panel wall, desirably the adsorbent is heated sufficiently to set or cure the binder and thereby bind the adsorbent to the panel. If the adsorbent is a zeolite, the heating also serves to liberate water adsorbed by the zeolite molecular sieve. See column 3, lines 50-62. The adsorbent may be mixed with the binder to form an aqueous slurry (e.g., column 4, lines 35-38). Gases that can be adsorbed include water, carbon dioxide, and vaporized organic liquids (column 5, lines 1-4).
U.S. Pat. No. 4,036,360 concerns a package having a desiccant composition. This patent refers to prior art packages at column 1, lines 22-41, including one that uses microporous polyurethane bonding a nylon mesh to form a sheet material (U.S. Pat No. 3,326,810). This patent uses prepolymerized polyurethanes to bind large quantities of desiccants such as zeolites (column 2, lines 7-39). Other organic resin can be mixed with the polyurethane (column 3, lines 10-18). Example 1 shows tetrahydrofuran mixed with polyurethane and silica gel and then coated onto polyester film.
U.S. Pat. No. 4,769,053 (assigned to Semco) concerns total enthalpy air-to-air rotary energy exchangers, also known as total heat wheels, and total heat exchange media employed in those wheels. A layer of coating composition comprising a molecular sieve material is applied to at least a portion of the surface of the sensible heat exchange material. The substrate may be a foil material of, e.g., aluminum, stainless steel, kraft paper, nylon fiber paper, mineral fiber paper, asbestos, or plastic (column 4, lines 56-61). The heat exchange media (molecular sieve material) adsorbs water but not contaminants, such as hydrocarbons, carbon monoxide, nitrogen dioxide, and sulfur dioxide (column 3, lines 18-30). Suitable molecular sieve materials are described at column 5, line 4, to column 6, line 41, and preferably have a pore diameter of about 3 Angstroms. Suitable binders are set forth at column 6, lines 41-58, and include polyurethanes, nitrile-phenolics, water-based binders, and alkyd-based resins. The binder composition preferably includes a solvent such as toluene (column 6, lines 58-61). Methods of making the heat exchange media are set forth at column 6, line 42, to column 7, line 19. The binder and molecular sieve material should be applied so that the binder does not block the pores of the molecular sieve, which would destroy the ability of the molecular sieve to function (Id.).
U.S. Pat. No. 5,052,188 concerns a process for reducing the polarity on the internal surfaces of various zeolites having an SiO.sub.2 to Al.sub.2 O.sub.3 ratio of at least about 3 and an average pore diameter size within the range of from about 4 to about 10 Angstroms. The modified zeolites are prepared by heating the starting zeolite in an aqueous medium also containing an acid or a source of ammonium ions to at least partially dealuminize the zeolite and thereby increase the ratio of silicon to aluminum present in the tetrahedral structure. The process also provides for the hydrogen ion exchange with respect to those zeolites that contain significant amounts of metallic cations in the structure, thereby replacing the bulky metallic cations with less bulky hydrogen ions, which in turn increases the water adsorptive capacity of the zeolite. Achievement of the appropriate equilibrium between reduced surface polarity and increased sorptive capacity is said to yield zeolite materials having an isotherm with a separation factor within the range of from about 0.07 to about 0.1. Those modified zeolites are said to be ideal desiccants for gas-fired air conditioning and dehumidification systems, for example, systems using regeneratable rotary desiccant wheels.
U.S. Pat. No. 5,120,694 concerns a method of coating an aluminum substrate (e.g., a foil) with a solid adsorbent (e.g., silica gel or a molecular sieve) comprising heating the surface of the substrate, contacting the surface with a slurry containing the adsorbent and a binder, and heating the coating to form a hardened surface. Suitable binders include clay (column 5, lines 8-30). The slurry may contain a dispersing agent or surfactant to aid in suspending the particles or to vary the slurry viscosity, e.g., a polymeric carboxylic acid or tetrasodium pyrophosphate (column 6, lines 5-15). The suspending liquid for the slurry is preferably water (column 6, lines 16-43). The coated product may be used in a desiccant wheel for cooling, refrigeration, and dehumidification (column 9, lines 20-29).
Rotary air-to-air total energy exchangers may be used in the HVAC field to recover both sensible energy (from a temperature change) and latent energy (from adsorbing water) from an exhaust air stream and then exchange these with an incoming air supply stream. The ability to recover the latent energy is of significant interest because such recovery occurs when, and as a result of, dehumidifying the outdoor air during a cooling cycle and from humidifying the outdoor air during a heating cycle, thereby reducing the energy demands required to condition outdoor air during those cycles.
The rotary wheel in such a total energy recovery system typically rotates at about 20 revolutions per minute and is commonly a thin substrate (e.g., a 2-mil thick aluminum foil) coated on both sides with a particulate desiccant in a binder matrix (typical coating thickness of about 1 mil on each side). Because the primary function of such a wheel is to recover both energy and moisture, because the desiccant readily picks up moisture and has a relatively low heat capacity, and because the substrate readily picks up heat but not moisture, the mass of desiccant in such a wheel is relatively low (about 15-30% of the total wheel mass) and the mass of the substrate (e.g., aluminum) is relatively high (about 70-85% of the total wheel mass). Additionally, the speed of revolution is necessarily high relative to the flow of air being processed to increase the rate at which heat and mass can be transferred from one air stream to the other air stream.
In contrast, a rotary wheel used for dehumidification only and not for total energy recovery has relatively less substrate mass (40-50%), relatively more desiccant mass (50-60%), and rotates more slowly (e.g., 0.25 revolutions per minute). That increases the amount of water that can be adsorbed and reduces the amount of carry-over heat that is transferred to the cooler air stream. A desiccant used for such a wheel desirably has as high a water adsorption capacity as possible and as much desiccant mass on the wheel as is consistent with technical and economic constraints (desirably, coating thicknesses of more than 1 mil). Furthermore, although some non-desiccant mass must be used to carry and support the desiccant (i.e., the substrate and the binder), the wheel should have as little non-desiccant mass as possible because such mass is dead weight and reduces the wheel's dehumidification efficiency and increases the energy required for regeneration.
Regardless of the type of wheel or other desiccant monolith (i.e., structural unit comprising the substrate carrying the desiccant particles) used or desiccant-based system in question, the binder holding the desiccant particles to the substrate should not significantly interfere with the functioning of the desiccant (e.g., should not occlude the pores of the desiccant or otherwise adversely affect its adsorptive or desorptive capabilities), should facilitate formation of the monolith (e.g., make coating the surface of the substrate with desiccant easy), should adhere to the desiccant tightly (to prevent loss of desiccant from the binder-desiccant coating layer, for example, by dusting), should present a readily cleanable surface, and should adhere the binder-desiccant coating layer tightly to the substrate. The binder must also function under the specified operating conditions, e.g., in the specified thermal and chemical environment. For example, a desiccant-coated total heat wheel is required to operate at temperatures of up to only about 100 degrees Fahrenheit (about 38.degree. C.). In contrast, a desiccant-coated dehumidification wheel should not be adversely affected by temperatures up to about 350 degrees Fahrenheit (about 177.degree. C.) and must be able to be repeatedly cycled between first temperatures in the range of 50 to 100 degrees Fahrenheit (about 10.degree. to 38.degree. C.) and second temperatures in the range of 300 to 350 degrees Fahrenheit (about 149.degree. to 177.degree. C.) without any adverse consequences, e.g., delamination of the binder-desiccant coating from the substrate.
Some early dehumidification wheels utilized a honeycomb paper impregnated with sodium silicate to form a backbone, which was then impregnated with a desiccant. Because absorbent desiccants such as lithium chloride, calcium chloride, and lithium bromide deliquesce and change from solid to liquid upon saturation, this type of desiccant could be easily deposited into the paper backbone by dipping the honeycomb wheel into a solution of the desiccant.
However, a significant problem with this type of desiccant was its loss from the wheel if the desiccant was allowed to reach saturation, although that usually could be avoided because of the high absorption capacity of such compounds (they can hold up to twice their own weight in water). Even so, problems occurred when such wheels became wet, came into contact with high humidity, or came into contact with pollutants such as sulfur dioxide and nitrogen dioxide. Also, manufacturing such wheels required numerous steps, including forming the special paper, winding and corrugating the paper to form the honeycomb, forming a silicon dioxide backbone by dipping the honeycomb into an aqueous sodium silicate solution, heating to drive off the water, impregnating with desiccant (e.g., LiCl) in a water bath, heating to drive off the water, grinding the wheel surface flat to open plugged flutes of the honeycomb, and hardening the surface. Use of that manufacturing procedure made mass production difficult and increased cost.
An advance over wheels utilizing absorbent desiccants is the use of solid adsorbents such as silica gel, activated alumina, and molecular sieves because they are chemically stable and do not deliquesce. Because solid adsorbents adsorb water in an amount equal to only a fraction of the their own weight, wheels using such desiccants must carry significantly more adsorbent mass than the earlier wheels (e.g., four to six times as much desiccant mass). To accommodate this much higher desiccant mass, some current dehumidification wheels are made from sheets formed using papermaking equipment from a mixture of pulp, desiccant, and binder in which the desiccant becomes an integral part of each sheet. However, sheets containing 50% or more desiccant (a desiccant wheel having acceptable performance needs at least 50% of its mass to be active desiccant) are difficult to form into honeycomb media and must be handled carefully because of decreased web strength resulting from the high desiccant loading. This makes mass production difficult and increases costs.
Other current dehumidification wheels utilizing solid adsorbents are made by preparing special paper, winding and corrugating the paper to form the honeycomb wheel, impregnating with sodium or ethyl silicate, converting the silica to silica gel using an acid or base, heating to dry the silica gel backbone and eliminate organic materials, grinding the wheel surface flat to open plugged flutes of the honeycomb, and hardening the surface. However, the dipping steps result in uneven film coatings and limit the amount of active desiccant that can be deposited on the wheel. Furthermore, the multi-step process is complex and makes the wheels costly to prepare.
Thus, there is a continuing need for efficient desiccant-based dehumidification wheels that can be easily and economically produced using environmentally lower-impact production techniques (e.g., without organic solvents), for desiccant-coated substrates that can be used to make those wheels, and for desiccant-coated substrates in general in which the binder matrix of the coating is sufficiently breathable so that the material(s) to be adsorbed can reach the desiccant particles quickly enough, in which the desiccant particles in the coating have a high percentage of their original capacity to adsorb and desorb, and in which the desiccant-coated substrate has sufficient flexibility and the coating has sufficient adherence to the substrate so that the desiccant-coated substrate can be formed into shapes having abrupt radii without the coating losing its integrity or its adherence to the substrate. There is also a need for desiccant-coated substrates that have thick coatings (i.e., coatings 2 mils thick or more per side) and in which the desiccant particles constitute a high percentage by weight of the coating. There is also a need for desiccant-coated substrates that have thick even coatings, i.e., a coating that does not vary significantly in its thickness along a given substrate. There is also a need for desiccant-coated substrates that can be used at temperatures above 150 degrees Fahrenheit (about 66.degree. C.), preferably above 200 degrees Fahrenheit (about 93.degree. C.), and particularly for substrates that can be repeatedly cycled during use between first temperatures in the range of 50 to 100 degrees Fahrenheit (about 10.degree. to 38.degree. C.) and second temperatures in the range of 300 to 350 degrees Fahrenheit (about 149.degree. to 177.degree. C.). There is also a need for desiccant-coated substrates in which the desiccant comprises two or more different desiccants that desirably are as well-intermixed (i.e., homogeneous) as possible in the coating layer. There is also a need for devices for air treatment besides dehumidification wheels that utilize such desiccant-coated substrates. Finally, there is a need for cost-effective methods for making such substrates and dehumidification wheels and other air treatment devices.